Electrically conductive polymer composition

ABSTRACT

A polymer composition comprises a solid fusible particulate polymer material and a quantity of metal particles that have been blended with the particles of polymer material to render the composition electrically conductive. The composition may be consolidated by subjecting the particulate blend to heat and/or pressure. The electrically conductive polymer composition may be used to connect the electrically conductive screen (7) of a dimensionally recoverable article (6) to the electrically conductive shield (4) of a cable (1).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polymer compositions and especially toelectrically conductive compositions.

2. Introduction to the Invention

Electrically conductive adhesives are employed in a number of fieldswhere good electrical continuity is required. For example, when ashielded electrical cable is terminated with an electrical connector, ascreened enclosure, e.g.: in the form of a dimensionally recoverablearticle, may be located over the connector and cable to provide acontinuous screening between the connector and the cable shield. Sucharticles are described in U.S. Pat. No. 4,467,002, the disclosure ofwhich is incorporated herein by reference. It is important to maintain agood electrical connection between the screen and the enclosure and boththe connector and the cable shield, and electrically conductiveadhesives have been employed for this purpose.

Electrically conductive adhesives can be formed by blending a quantityof electrically conductive material, e.g. a metal, into a solutions ofthe adhesive in a suitable solvent and evaporating the solvent. Normallysilver flake is employed for this purpose in view of the relatively lowbulk resistivities that can be achieved. The bulk resistivity of theadhesive will fall sharply with the increase in volume loading of metaluntil a so-called "percolation threshold" is reached after which theresistivity is low and relatively constant with respect to the metalloading. The silver loading employed will depend on the type ofadhesive, but loadings in the region of 12-15% by volume may be used,may be used.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an adhesivecomposition which comprises a solid fusible particulate polymer materialand a quantity of metal particles that have been blended with theparticles of polymer material to render the composition electricallyconductive.

Preferably the metal particles have an aspect ratio (i.e. the ratio oftheir largest to smallest dimension) of at least 5 and especially atleast 10, the particles preferably being in the form of flake.

We have observed that the metal flakes adhere to the surface of thepolymer particles, which is at least partly due to tackiness of thepolymer particles, so that when the adhesive is fused the metal flakesform a relatively nonuniform dispersion on a microscopic scale, incontrast with previously used adhesives in which a very uniformdispersion is obtained. On fusion of the adhesive particles, the metalflakes tend to remain in the same regions as before fusion, therebyforming a large number of metal-flake-rich conductive paths throughoutthe fused or resolidified composition. It has been found that thequantity of metal flake required in the composition to reach thepercolation threshold can, at least in some cases, be significantlyreduced. In addition it is possible to use the composition to formadhesive bonds whose electrical conductivity remains relatively constantover long periods of time at elevated temperatures.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated by the drawing in which, FIG. 1 is asection through an electrical cable of the invention;

FIGS. 2 to 4 are graphs of the bulk resistivity of compositions of theinvention and comparative compositions as a function of the loading ofsilver particles in the composition;

FIG. 5 is a graph of the resistance of an adhesive composition of theinvention as a function of storage time at different temperature; and

FIG. 6 is a graph of the resistance of an adhesive composition of theinvention as a function of storage at elevated temperature.

The composition according to the present invention may be employed for avariety of purposes. For example, it may be employed as an adhesivewhere an electrically conductive adhesive joint is required. In such acase the polymer composition may for example be a hot-melt adhesive or acurable adhesive. In other cases the composition may be employed as avoid filler or mastic.

Preferably, the adhesive composition includes not more than 12% byvolume of the metal particles based on the total volume of thecomposition.

As hot-melt adhesives those based on olefin homo- or copolymers may beused e.g., low, medium or high density polyethylene, polypropylene,ethylene/vinyl acetate or ethylene/methyl acrylate copolymers may beemployed. Alternatively the adhesive may be formed from a polyester,preferably one based on a polyalkylene diol having at least 3 carbonatoms or a cycloaliphatic diol, and an aromatic dicarboxylic acid, or apolyamide e.g., as described in in U.S. Pat. No. 4,181,775. In the caseof hot-melt adhesives the adhesive preferably includes not more than11%, more preferably not more than 10%, and especially not more than 9%by volume of metal particles based on the total volume of the conductiveadhesive. Indeed, it is possible to form satisfactory adhesives with 5%or less by volume of metal particles or even 4% or less.

Normally the electrical resistance of the adhesive increases sharply asthe loading of the metal particles is reduced below the percolationthreshold. Thus, the loading of the metal particles should be above thisand will normally be at least 1.5% by volume.

The invention has the further advantage that it enables the manufactureof electrically conductive hot-melt adhesives based on polymers such asethylene/vinyl acetate polymers which are not soluble in common solventsand so cannot be blended with the metal particles in solution.

Where ethylene copolymers are employed acidic copolymers are preferred.Polymers which have an acid number (expressed in mgKOH/g) of at least 5,but preferably less than 200 and especially less than 100 (such asacidic ethylene vinyl acetate polymers and ethylene isobutyl acrylate,methacrylate acid terpolymers) may exhibit favorable electricalstability.

As curable or thermosetting adhesives, those described in U.S. Pat. No.4,707,388 to Park et al may be used, the disclosure of which isincorporated herein by reference. These adhesives will cure by reactionof a plurality of mutually reactive components which exist separatelyfrom one another in the form of particles, and which will cure once theadhesive has been heated to fuse the particles. The adhesive ispreferably based on an epoxy and a curing agent, for example apolyamide, carboxylic anhydride, phenolic resin or blocked isocyanatecuring agent.

The composition may include one or more other components such asfillers, pigments, antioxidants and the like or agents for subsequentcross linking. Preferably it includes a binder, preferred bindersincluding water-soluble polymers e.g. polyalkylene oxide, and latices.The binder is preferably present in the range of up to 20% by weight andespecially from 1 to 10% by weight, based on the weight of the adhesivecomposition.

Where a curable adhesive is used it preferably includes not more than15%, more preferably not more than 13%, and especially from 5 to 12% byvolume of metal particles, based on the total volume of the conductiveadhesive composition.

Preferably substantially all the particles have a size in the range offrom 10 to 1000 micrometers. The preferred particle size will depend onthe type of adhesive, at least to some extent. Preferably the particlesize is at least 100 micrometers and especially at least 180micrometers.

Preferably it is not more than 600 micrometers and especially not morethan 300 micrometers. The metal particles should be sufficiently smallerthan the polymer particles to enable them to coat the surface of thepolymer particles. Preferably the ratio of the weight average size ofthe adhesive particles to that of the metal particles is in the range offrom 10:1 to 200:1 and especially from 20:1 to 100:1.

Preferably the polymer material has a melt index of less than 100.

This invention also provides a dimensionally recoverable article whichis coated on at least part of its internal surface with such an adhesivecomposition. The article may contain an electrically conductive screen,for example to provide e.m.i. shielding to a cable connection, in whichcase the conductive composition is preferably located in contact withpart of the screen for forming an electrical connection between thescreen and a substrate, e.g., a cable screen or connector about whichthe article is recovered, for example as described in U.S. Pat. No.4,467,002 mentioned above. The curable adhesive or hot-melt adhesive mayequally be employed on the end of the article intended to be recoveredon the connector back shell and on the other end intended to berecovered about the cable. The conductive adhesive may be employed incombination with a conventional hot-melt or curable adhesive which, forexample, could be located on that part of the article intended to berecovered over a cable jacket.

The adhesive composition need not be employed solely with dimensionallyrecoverable articles but may be used in numerous places where anelectrically conductive adhesive joint is required. Thus the inventionalso provides an electrical cable which has been terminated by means ofa connector and an enclosure that extends from the connector to thecable and has an electrically conductive screen, the electricallyconductive screen and the cable shield being electrically connected bymeans of a polymer composition as described above.

Surprisingly we have found that it is possible to consolidate thecomposition by application of heat and/or pressure, preferably withsufficient heat and pressure to fuse the polymer particles and to forcethe composition into the desired shape, without any loss, or anysignificant loss in the electrical resistivity of the composition. Thus,according to another aspect, the invention provides a method of formingan electrically conductive adhesive composition which comprises:

(i) blending a solid fusible particulate polymer material with aquantity of metal particles to form a particulate blend; and

(ii) subjecting the particulate blend of step (i) to heat and/orpressure to consolidate the polymer material.

The particulate composition according to the invention may be formedinto a tape or sheet by heat and pressure, and in this form, may beemployed for a number of purposes. For example a sheet formed from acurable material (that has cured during pressing) may be used as aconductive gasket. A tape or sheet formed from a hot-melt adhesive maybe employed with a dimensionally recoverable article in the same manneras the particulate composition described above. In this case the tape orsheet may be wrapped to form a cylinder of the appropriate diameter andthe article may be partially recovered about it so as to grip it.

The inhomogeneity of the adhesive after consolidation can be observedand determined by analysis of photomicrographs of the composition. Theinhomogeneity can be quantified as a "degree of inhomogeneity" by meansof the method described in Example 4 below. Thus, according to yetanother aspect, the invention provides a polymer composition whichcomprises a sold fusible polymer material and a quantity of metalparticles that have been blended with the particles of polymer materialto render the composition electrically conductive, the compositionhaving a degree of inhomogeneity of at least 400 μm² ; preferably atleast 500 μm² and especially at least 600 μm².

The invention will now be described by way of example with reference toFIG. 1 which is a section through a cable termination and articlerecovered thereon.

Referring to FIG. 1, a cable 1 is terminated with a connector 2, andcomprises wires 3, braid 4 for screening the cable and jacket 5. Adimensionally recoverable article or so-called "boot" 6 has beenrecovered about the connector and proximal end of the cable 1 to enclosethe assembly. The boot 6 is provided with an internal tin coated coppere.m.i. screen 7 that extends substantially the whole length of the boot,the screen having a number of longitudinally extending corrugations orflutes that allow it to contract in circumference and accommodaterecovery of the boot.

The unrecovered boot 6 is provided with a ring of particulate conductiveepoxy adhesive 8 at the connector end, the adhesive being located on theinternal surface of the end portion of the screen 7. In addition theboot is provided with a ring of particulate conductive hot-melt adhesive9 and in the region of the other end, this ring also being located onthe internal surface of the end portion of the screen 7. A second ringof conventional or particulate but electrically nonconductive adhesive10 is provided between the adhesive 9 and the outlet of the boot.

In order to enclose the assembly, the boot 6 is slipped over it andpositioned with engagement lip 11 in alignment with a correspondingannular recess 12 in the connector adaptor 2 of the connector. The boot6 is then heated, for example, by means of a hot-air gun, until itrecovers about the spin-coupling adaptor 2. As the boot recovers theepoxy adhesive 8 melts and enables a conductive bond to be formedbetween the spin coupling adaptor 2 and the boot 7. At the same time orat a later stage the other end of the boot 6 is recovered about thecable. As this end of the boot is heated the conductive hot-meltadhesive 9 and the conventional hot-melt adhesive 10 melt, theconductive adhesive 9 forming a bond with the braid 4 of the cable andthe conventional hot-melt adhesive 10 forming a bond to the cablejacket.

The following Examples illustrate the invention:

EXAMPLE 1

The following hot-melt adhesive components:

    ______________________________________                                                           Parts by weight                                            ______________________________________                                        Polyamide    Unirex 2647 90                                                   Polyamide    Macromelt 6156                                                                            10                                                   ______________________________________                                    

were cryogenically ground to a particle size of 106 to 600 micrometers(with approximately two thirds the particles of particle size less than300 micrometers) and blended together. To this various quantities ofsilver flake (Johnson Matthey FS2) of particle size 4 micrometers wereincorporated and dry blended. 8.5 parts by weight of polyethylene oxidebinder were blended into the resulting mixture. A slurry of the blendwas then formed in approximately 100 parts water.

The bulk resistivity of the adhesive was determined by depositing a 2 mmthick layer of the slurry onto a tinned copper plate, evaporating thewater at 40° C. for 24 hours, placing a further tinned copper plate onthe dried adhesive, and heating the assembly to 150° to 200° C. for 10to 20 minutes under a 300 g weight.

The adhesive exhibited the resistivity values given in table I and showngraphically in FIG. 2.

COMPARATIVE EXAMPLE 1

The same polyamide components as in Example 1 were dissolved indichloromethane and mixed together, after which various quantities ofthe silver flake were added. The dichloromethane solvent was thenremoved by evaporation at 40° C. for 24 hours. The resulting materialwas cut into strips and formed into a plaque.

The bulk resistivity of the plaqued material was determined bysandwiching the plaque between two tinned copper plates and heating theassembly as described in Example 1.

The results are given in table I and shown graphically in FIG. 2.

                  TABLE I                                                         ______________________________________                                                   Example 1   Comparative Example 1                                  % Silver   Bulk Resistivity                                                                          Bulk Resistivity                                       (by volume)                                                                              10.sup.-5 ohm m                                                                           10.sup.-5 ohm m                                        ______________________________________                                        12.14      4                                                                  12                     3                                                      10                     5.6                                                    8.4        3                                                                  8                      4.06                                                   6.5                    10156                                                  4.2        5.3                                                                2.2        27                                                                 1.1        412                                                                ______________________________________                                    

EXAMPLE 2

Example 1 and comparative Example 1 were repeated with the exceptionsthat the polyamide adhesives were replaced by an acid functionalisedethylene/vinyl acetate hot-melt adhesive terpolymer (CXA 2002 from DuPont) and xylene replaced dichloromethane as solvent. The bulkresistivity was determined as in Example 1 and the values obtained aregiven in table II and values of the non-comparative Example are shown inFIG. 3.

                  TABLE II                                                        ______________________________________                                                   Example 2   Comparative Example 2                                  % Silver   Bulk Resistivity                                                                          Bulk Resistivity                                       (by volume)                                                                              10.sup.-5 ohm m                                                                           10.sup.-5 ohm m                                        ______________________________________                                        14.32      5.625                                                              12.5                   2.18                                                   11.17      5.4                                                                10.0                   10.63                                                  7.73       4.06                                                               7.5                    11884                                                  5.91       5.66                                                               4.02       7.56                                                               2.05       36.5                                                               1.0        1734                                                               ______________________________________                                    

EXAMPLE 3

Example 1 was repeated with the exception that the polyamide adhesivecomponents were replaced by the following reactive components:

    ______________________________________                                                       Trade name                                                                              Parts by weight                                      ______________________________________                                        Epoxy resin based on                                                                           DER 662     77                                               bisphenol A                                                                   Dimerised fatty acid reactive                                                                  Macromelt   20                                               polyamide        6071                                                         Dimethylaminopyrodine         3                                               accelerator                                                                   Polyethylene Oxide            4                                               binder                                                                        ______________________________________                                    

The components were cryogenically ground and the particles were gradedso that no particles were greater than 300 micrometers in size.

The bulk resistivity was determined as in Example 1, and the results aregiven in table III.

                  TABLE III                                                       ______________________________________                                        % Silver       Bulk Resistivity                                               (by volume)    10.sup.-5 ohm m                                                ______________________________________                                         4             120                                                             9             31                                                             12             18.7                                                           15             17                                                             20             18.7                                                           ______________________________________                                    

EXAMPLE 4

The EVA terpolymer adhesive and silver flake employed in Example 2(without binder) were cryogenically ground to particle size of 106 to600 micrometers and were dry blended together. The powered blend wasthen formed into a plaque at 120° C. over a period of about 2 minutes.The bulk resistivity of the plaqued material was determined as incomparative Example 1, and the results are given in table IV and givengraphically in FIG. 5.

                  TABLE IV                                                        ______________________________________                                        and shown graphically in FIG. 5                                               % Silver       Bulk Resistivity                                               (by volume)    10.sup.-5 ohm m                                                ______________________________________                                        11.17          3.43                                                           9.45           4.47                                                           7.73           2.8                                                            5.91           8.02                                                           4.02           6.45                                                           2.05           8625                                                           ______________________________________                                    

A photomicrograph of a section through the plaqued material was taken,so that the distribution of silver particles within the material couldbe analyzed. A magnification of X600 was used. The photomicrograph thusobtained consisted of black regions corresponding to silver and whiteregions corresponding to the EVA terpolymer adhesive. Thephotomicrograph was then scanned using an optical line scanner and theinformation thus obtained from it was digitally stored by computer. Thescanning resolution was 4.41×10⁻⁸ mm² per bit of information (that is,each area of the photomicrograph corresponding to an area of 4.41×10⁻⁸mm² of the sample was stored in a file as being either black or white).

Each line, (of bits of information) corresponding to a row of squares ofarea 4.4×10⁻⁸ mm² of the photo micrograph was then analyzed. For eachline, the largest number of consecutive bits of informationcorresponding to silver (i.e. the largest unbroken region of silver inthe line) was measured and converted into units of micrometers (referredto as W). Also, for each line,. the number of discrete unbroken regionsof silver per micrometer was measured (referred to as H and having unitsof counts per micrometer).

For each line, the value of W/H, measured in μm², was calculated and thebest overall value of W/H for all the lines was determined using themethod of least squares. This value is referred to as the "degree ofinhomogeneity" and is used as a measure of the inhomogeneity of theplagued blended material. The value for the degree of inhomogeneityobtained from the sample used in this example was 730 μm². The degree ofinhomogeneity of a comparative sample prepared in accordance withComparative Example 2 was determined by this method and was found to be180 μm².

A 10 mm wide strip of the adhesive so formed was wrapped around a coppertube, and a dimensionally recoverable article as described in U.S. Pat.No. 4,467,002 was then recovered onto the adhesive strip. The assemblywas heated to one of three temperatures, 100°, 125° and 150° C. for upto about 50 days, and the d.c. resistance of the adhesive bond wasmeasured at various times. The results are shown graphically in FIG. 5from which it can be seen that the bond is relatively insensitive toheat aging in this manner. The fact that the bond resistance was low andstable was not expected since the test temperature was considerablyhigher than the softening point of the adhesive (70°-75° C.).

A dimensionally recoverable screened moulded part according to U.S. Pat.No. 4,467,002 was recovered onto a harness having a metal braid andhaving a 10 mm wide strip of the adhesive so formed wrapped around thebraid. The other end of the moulded part was recovered about a connectoradaptor and bonded thereto by means of a silver flake loaded two partepoxy adhesive. The d.c. resistance was measured between the adaptor andthe braid while the article was held at 75° C., i.e. just above thesoftening point of the adhesive. The results are shown in FIG. 6 inaddition to the results obtained using a silver loaded two part epoxyadhesive. It can be seen that the results for the two adhesives are notsignificantly different and that the EVA adhesive exhibits a relativelystable conductivity at this temperature over a period of 200 days.

We claim:
 1. An adhesive composition which comprises(a) a solid fusibleparticulate thermoplastic polymer material and (b) at least 4% by volumeof metal particles that have been blended with the particles of polymermaterial to render the composition electrically conductive, thecomposition having (i) been consolidated by heat and/or pressure, (ii) adegree of inhomogeneity of at least 400 μm², and (iii) a resistivity notmore than about 8×10⁻³ ohm-cm.
 2. A composition as claimed in claim 1,wherein the metal particles have an aspect ratio of at least
 5. 3. Acomposition as claimed in claim 1, wherein the metal particles compriseflake.
 4. A composition as claimed in claim 1, which includes not morethan 12% by volume of the metal particles based on the total volume ofthe composition.
 5. A composition as claimed in claim 1, wherein thepolymer material is a hot-melt adhesive.
 6. A composition as claimed inclaim 5, wherein the adhesive comprises an ethylene/vinyl acetatecopolymer, a polyester or a polyamide.
 7. A composition as claimed inclaim 5, which includes not more than 9% by volume of the metalparticles.
 8. A composition as claimed in claim 1, which includes from 5to 12% by volume metal particles.
 9. A composition as claimed in claim1, wherein the ratio of the weight average size of the particles ofpolymer material to the weight average size of the metal particles is inthe range of from 10:1 to 200:1.
 10. A composition as claimed in claim 9wherein the ratio is in the range of from 20:1 to 100:1.
 11. Acomposition as claimed in claim 1, wherein the polymer material has amelt index of less than
 100. 12. A composition as claimed in claim 1which has been formed by a process which comprises(i) blending theparticulate thermoplastic polymer material with the metal particles toform a particulate blend, and (ii) subjecting the particulate blend ofstep (i) to heat and/or pressure to consolidate the polymer material,said composition being in the form of a tape or sheet.
 13. A compositionaccording to claim 1 which has a resistivity of about 3×10⁻³ to about8×10⁻³ ohm-cm.
 14. An adhesive composition which comprises(a) a solidfusible particulate thermoplastic polymer material and (b) at least 4%by volume of silver particles that have been blended with the particlesof polymer material to render the composition electrically conductive,the composition having (i) been consolidated by heat and/or pressure,(ii) a degree of inhomogeneity of at least 400 μm², and (iii) aresistivity not more than about 8×10⁻³ ohm-cm.
 15. A compositionaccording to claim 14 wherein the silver particles have an aspect ratioof at least
 5. 16. A composition according to claim 14 wherein thesilver particles comprise flake.
 17. A composition according to claim 14which includes 5 to 12% by volume of silver particles.
 18. A compositionaccording to claim 14 wherein the ratio of the weight average size ofthe particles of polymer material to the weight average size of thesilver particles is in the range of from 10:1 to 200:1.
 19. Acomposition according to claim 14 wherein the polymer material is ahot-melt adhesive.